1. Field of the Invention
The present invention relates to a complementary metal oxide semiconductor (CMOS) variable gain amplifier (VGA). More particularly, the present invention relates to a CMOS VGA for dB-linearly controlling an output gain.
2. Description of the Related Art
Complementary metal oxide semiconductors (CMOSs) constituting mobile communication systems such as ultra wide band (UWB) systems are required to operate at a low power and to be compact. VGAs are necessarily used to maximize a dynamic range of an entire mobile communication system and vary a gain of a wide operation area so as to have a stable signal characteristic. In detail, such a VGA operates at a low voltage to consume a low power so as to uniformly maintain an output signal for an input range of an input signal more widely varying. As a result, the linearity of an output gain is improved and increased.
However, a CMOS VGA according to the prior art requires an additional exponential function generator to have a gain of an exponential function so as to control an output gain on a dB linear scale. For instance, a Gilbert Cell structure including a CMOS VGA suggested in “Design of Analog CMOS Integrated Circuits” by Razavi includes an exponential function generator.
FIG. 1 is a block diagram of an example of a CMOS VGA according to the prior art. Referring to FIG. 1, the CMOS VGA has a Gilbert Cell structure. The CMOS VGA having the Gilbert Cell structure includes an exponential function generator and a VGA core cell. An entire area of the CMOS VGA is 740 um*540 um, and the exponential function generator occupies an area of 220 um*540 um of the entire area.
FIG. 2 is a detailed circuit diagram of the CMOS VGA shown in FIG. 1. Referring to FIG. 2, the CMOS VGA increases a dynamic range of an input signal using a degenerate resistor M63. However, currents of transistors M61 and M62 must sharply vary to dB-linearly vary a gain of the entire CMOS VGA. Thus, the exponential function generator is required.
Accordingly, when a CMOS VGA is realized, a wide area is wasted by an exponential function generator. Thus, the CMOS VGA cannot be compact. Also, a large amount of power is consumed. As a result, the CMOS VGA cannot consume a small amount of power and operate at a low voltage.